show Abstracthide AbstractIn this study, we conducted a light-dark experiment and a comprehensive comparative transcriptomic study in Fragum sueziense, a photosymbiotic bivalve, and a closely related non-symbiotic species, Trigoniocardia granifera. Our study revealed ample molecular pathways involved in photosymbiosis, and many were shared between mollusks and other distantly related host lineages. This provides important insights into metazoan symbiosis evolution, indicating ancient gene repertoires and parallel evolution play important roles in the independent evolution of photosymbiosis in animals.